This phase I trial studies the side effects and best dose of trametinib when given together with fluorouracil and radiation therapy before surgery in treating patients with stage II-III rectal cancer. Trametinib may stop the growth of tumor cells by blocking some of the enzymes needed for cell growth. Drugs used in chemotherapy, such as fluorouracil, work in different ways to stop the growth of tumor cells, either by killing the cells or by stopping them from dividing. Radiation therapy uses high-energy x-rays to kill tumor cells. Giving trametinib together with fluorouracil and radiation therapy before surgery may make the tumor smaller and reduce the amount of normal tissue that needs to be removed